Follow up for D53051

This patch introduces the tool associated with the ELF implementation of
TextAPI (previously llvm-tapi, renamed for better distinction). This
tool will house a number of features related to enalysis and
manipulation of shared object's exposed interfaces. The first major
feature for this tool is support for producing binary stubs that are
useful for compile-time linking of shared objects. This patch introduces
beginnings of support for reading binary ELF objects to work towards
that goal.

Added:

 - elfabi tool.
 - support for reading architecture from a binary ELF file into an
 ELFStub.
 - Support for writing .tbe files.

Differential Revision: https://reviews.llvm.org/D55352

llvm-svn: 350341